Vehicle powertrains typically include a torque converter positioned between the engine and the transmission. The torque converter is a fluid coupling which allows the engine to spin while the vehicle wheels and transmission gears come to a stop. If the engine is rotating slowly, such as when the car is idling at a stop light, the amount of torque passed through the torque converter is very small, so the car may be kept still with light pressure on a brake pedal. As the engine accelerates, the torque converter pumps increased amounts of fluid, thereby causing increased amounts of torque to be transmitted to the wheels for launching the vehicle.
Manual transmission vehicles use a clutch, which completely disconnects the engine from the transmission, to allow the engine to run while the vehicle wheels and transmission gears are at rest. In order to launch the vehicle, the clutch is slipped and gradually engaged to connect the rotating engine output with the transmission input, thereby moving the wheels and launching the vehicle.
Automatic transmissions sometimes eliminate the torque converter and replace it with a flywheel and isolator to achieve a friction launch configuration. In a friction launch configuration, one or more clutches inside the transmission are slipped and gradually engaged to gradually transfer torque from the engine into the transmission for launch.
Some automatic transmissions include a low/reverse braking clutch and a freewheeler to provide reaction torque in both forward and reverse directions as input clutches are slipped for launching the vehicle without a torque converter.